Electrostatic painting method and apparatus

ABSTRACT

Apparatus and method for electrostatically charging a stream of atomized paint particles discharged from a spray gun. An emitter electrode is positioned in spaced relation to the end of the spray gun to establish a corona discharge from the electrode tip to a conductive portion of the gun end so that the direction of the discharge is opposite to the direction of flow of the particle stream. The apparatus may comprise an attachment unit for converting conventional spray guns to electrostatic spray guns, including a dielectric flow path-defining member downstream of the spray gun.

Ilnited States Patent Inventors Meredith C. Gourdine West Orange; EdwardL. Collier, Morris Plains; Gerald P. Lewis, West Orange; Harold McCrae,Upper Montclair; Donald H. Porter, Carlstadt, all of NJ.

Appl. No. 771,135

Filed Oct. 28, 1968 Patented Oct. 19, 1971 Assignee Gourdine Systems,Inc.

Livingston, NJ.

ELECTROSTATIC PAINTING METHOD AND APPARATUS 22 Claims, 5 Drawing Figs.

US. Cl 239/3, 239/15 Int. Cl B05b 5/02 Field of Search 239/3, 15;

[56] References Cited UNITED STATES PATENTS 2,959,353 11/1960 Croskey etal. 239/15 2,995,393 8/1961 Charp 239/15 X 3,009,441 11/1961 Juvinall239/15 X 3,195,264 7/1965 Ward,.lr 239/3 X 3,292,860 12/1966 Williams etal. 239/15 Primary Examiner-Lloyd L. King Assistant Examiner-Gene A.Church Att0rneyBrumbaugh, Graves, Donohue & Raymond ABSTRACT: Apparatusand method for electrostatically charging a stream of atomized paintparticles discharged from a spray gun. An emitter electrode ispositioned in spaced relation to the end of the spray gun to establish acorona discharge from the electrode tip to a conductive portion of thegun end so that the direction of the discharge is opposite to thedirection of flow of the particle stream. The apparatus may comprise anattachment unit for converting conventional spray guns to electrostaticspray guns, including a dielectric flow path-defining member downstreamof the spray gun.

PATENTEDBU 19 I97! IOX 78 Fla 5 ///v/',/-J'/-/:5 MEREDITH C. GOURDINE,

I DONALD H. PORTER h4/(M1'l\lll7/)/ /14 ,41) "2 ,1 f 1mm their ATTORNEYSELECTROSTATIC PAINTING METHOD AND APPARATUS BACKGROUND OF THE INVENTIONThe present invention relates to electrostatic spray painting, andparticularly to an electrostatic spray painting apparatus and methodwherein atomized particles of paint, or other coating material, areprojected from a spray gun through a high-density corona discharge zonetoward the article to be coated.

Electrostatic spray guns for discharging a stream of charged atomizedpaint particles have been widely used in the paint industry. Such gunsusually include means for atomizing and discharging a stream of a paintinto the atmosphere either by impinging jets of compressed air on thepaint stream (air type), or by discharging the paint through a fineaperture under high pressure on the order 1,000 pounds per square inch(airless type). Normally, an electrostatic charge is imparted to theindividual paint particles concurrently with, or immediately after,their atomization. The charge transfer is typically accomplished bycontacting the atomized particles with an electrode at high potential orby passing them through a corona discharge extending transversely of thestream of particles. Usually the workpiece to be painted is maintainedat or near ground potential so that an electric field between the gunand the workpiece promotes attraction of the charged particles to theworkpiece.

Most known devices impress a direct current voltage between the emitter(corona) and attractor electrodes ranging between 40,000 to 100,000volts. High voltages of this order present several serious practicalproblems; for example, flashover sparking may occur between the emitterand attractor electrodes or between the emitter electrode and anadjacent conducting surface. Such sparking can result in fires andinjury to operating personnel. Moreover, high-potential apparatusrequire the use of substantial insulation and cumbersome resistors torender them electrically safe. Heavy conductors and large power packsare also needed to meet power consumption requirements of such systems.

High voltages have heretofore been considered essential, however, forthe reason that known electrifying apparatus, when operated at lowvoltages, did not produce a charge transfer to the individual particlessufficient to create the required charge density needed to producehigh-quality paint finishes. Additionally, most typical apparatus tendto bulkiness, lack of versatility and complexity. These and otherobjectionable features of the prior art are overcome by the presentinvention.

SUMMARY OF THE INVENTION In accordance with the present invention,electrostatic charging of the stream of coating particles isaccomplished at low-operating potentials. An emitter, or corona,electrode is positioned downstream from an attractor electrode, whichmay be the spray gun head, and a corona discharge having a substantialrearward component is established between the emitter and attractorelectrodes.

The emitter electrode preferably comprises a necdlelike member, the tipof which is disposed on or near the longitudinal axis of the particlestream. With the emitter electrode so located, it has been found that ahigh charge is transferred to the atomized paint particles, even whenthe discharge potential is as low as 8 to 12 kilovolts. With the coronadischarge being directed rearwardly, a high concentration of atmosphericions is produced which traverse the stream of atomized paint particlesin a direction opposite to the travel of the particles. Thisrelationship between the ionizing discharge field and the paint particlestream greatly increases the surface charge attainable by the individualatomized particles over that which occurs when the electric fieldextends either primarily transversely to or parallel with the stream.Since the atomized paint particles must travel longitudinally throughthe ion stream rather than perpendicularly across it, the effectiveresidence time of each particle in the ion stream is increased. Inaddition, some improvement in charging attributable to friction producedby ion-particle collision is also thought to exist. Both of thesefactors, the more friction-producing collisions and the longer exposuretime of the particle to the ionized region, tend to increase the chargetransferred to a given paint particle.

To minimize the turbulence introduced into the particle stream by theemitter electrode, and also to prevent the agglomeration of paintparticles on the electrode, the electrode may be mounted to extend intothe particle stream in the upstream direction at an angle with thelongitudinal axis of the stream of from 30 to 60. It also may be bentalong a predetermined radius of curvature to further reduce theformation of agglomerates on the electrode shank.

The electrical discharge from the needle tip can terminate at a separateattractor electrode, the air horns, spray nozzle, or other portion ofthe spray head. Downstream confinement of the charged particle streamfor the development of a higher space charge field, when required, isaccomplished through the use of a dielectric channel member receivingthe charged particle flow.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of these andother features of the invention, as well as the objects and, advantagesthereof, reference may be made to the following detailed description andto the drawing, in which:

FIG. 1A is a side elevation view, partly in section, of a basicelectrostatic charging apparatus in accordance with the presentinvention showing schematically the electrical circuit of the chargingapparatus;

FIG. 18 is an end view of the charging apparatus of FIG. IA;

FIG. 2 is a side elevation view, partly in section, of an exemplaryembodiment of the charging apparatus showing the charging apparatusmounted on an air-type spray gun;

FIG. 3 is a side elevation view, partly in section, of anotherembodiment of the charging apparatus showing the charging apparatusmounted on an airless-type spray gun; and

FIG. 4 is a schematic illustration, partly in cross section, showing theapplication of coating particles to workpiece surfaces by the chargingapparatus in accordance with the inventron.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT FIGS. IA and IB illustrateschematically an electrostatic charging apparatus in accordance with thepresent invention mounted on a conventional air-type spray gun 10 insurrounding relation to the spray head I2, spray nozzle I4 and air horns16. The air-type gun is shown for illustrative purposes only, thecharging apparatus being usable with equal facility with airless-typeguns as well.

The charging apparatus includes a high-voltage power supply 118, anemitter electrode 20 and an attractor electrode, comprising in FIGS. IAand 1B the sprayhead 12, including its component parts, the spray nozzle14 and'the air horns 16. As will be more fully described below,'theparticular component serving as the terminus for the electricaldischarge depends on the geometry of the gun end andits locationrelative to the emitter electrode 20. Since all or part of the sprayhead may function as the attractor electrode, and since it is desirablefor safety purposes to maintain the attractor electrode at groundpotential, the spray head 12 is grounded by a suitable conductor 21.

Although the charging apparatus is depicted in the drawing inconjunction with spray guns having spray heads which are electricallyconductive, and thus capable of functioning as attractor electrodes, itis to be understood that it is not essential that the gun spray head becomposed of a metallic or other conductive material. For example, thespray head can be made of a dielectric material and have a'conductiveelement either imbedded in the material or mounted on the head tofunction as the attractor electrode. Alternatively, the attractorelectrode can be carried as a separate element by a housing surroundingthe spray head. At least a portion of the attractor electrode, whateverits form, should be positioned rearwardly of the emitter electrode sothat a component of the corona discharge is directly in opposition tothe flow of the paint particle stream.

Voltages in the range of from 8 to 12 kilovolts are provided by thepower supply 18, which may also be grounded. A current limitingresistance 22 of suitable value is preferably inserted in the linebetween the power supply 18 and the emitter electrode 20 to reduce thepossibility of sparking and to establish correct loading of the circuitat the operating voltage.

A housing 24 comprised of a dielectric material, such as nylon, mountsthe emitter electrode 20 in proper relation to the spray head 12 and airhorns 16. The emitter electrode 20 may be either curved or straight, forreasons later set forth, and is in the form of an elongated needlelikewire. Preferably, the housing 24 is arranged on the gun so that the tip26 of the electrode is positioned substantially on the longitudinal axisof the atomized paint spray as it is discharged from the nozzle 14. Theelectrode tip 26 is spaced downstream of the nozzle 14 and horns 16 apredetermined distance which may be varied to position the tip 26 at theoptimum point as required by the magnitude of the voltage applied, thestructural geometry of the spray gun and the pattern of the atomizedpaint particle stream.

The tip 26 may be positioned even with or rearwardly of the ends of theair horns. Ordinarily, however, the tip 26 will be downstream of thehorns 16 and the nozzle 14. When an airless-type gun is used, theemitter tip is correspondingly spaced with respect to the nozzle 14 andspray head 12.

With the tip 26 in the position shown in FIG. 1A, and current suppliedto the emitter electrode, a continuous corona discharge is created,causing a high-density stream of atmospheric ions to flow in arearwardly and outwardly direction from the vicinity of the emitter tip26 to the spray head 12, the nozzle 14 and the horns 16. At closespacings of the tip 26 from the spray nozzle 14, the corona dischargemay extend principally between the tip and the nozzle 14, at least whenno paint is being discharged from the nozzle. Under normal operatingcircumstances, however, the corona discharge extending from the tip tothe nozzle is quenched by the flow of atomized paint particles so thatthe principal path followed by the ion stream is from emitter tip 26 tothe horns 16. In airlesstype guns, the ion stream normally extendsbetween the emitter tip and the spray nozzle when no paint is beingdischarged, and between the tip and both the spray nozzle and spray headwhen paint is being discharged.

In the representative embodiment shown in FIG. 2, the charging apparatuscomprises a converter unit, shown generally at 30, which attaches to aconventional, nonelectrostatic spray gun 32 to convert it to anelectrostatic spray painting device. The spray gun is similar to thatdescribed in connection with FIGS. 1A and 1B and, in addition, has aknurled collar 34 for removably mounting the spray head 36. Also, theconventional air horns 38 have inwardly directed openings 40 in theiradjacent faces through which air jets are passed to impinge against theatomized paint particles. The air jets prevent dispersed paint particlesfrom depositing on the horns and thus fouling the spray gun and alsoshape the paint particle stream into the long, narrow elliptical patterngenerally preferred for most spray painting operations.

The converter unit includes in general a housing 42, an annular mountingring 44, a channel member 46, all of which are composed of a suitabledielectric material, and an emitter electrode 48. Apertures 52 arecircumferentially spaced around the mounting ring 44, thereby formingbreather passages between the ring and housing. The converter unit 30 isattached to the gun by means of one or more setscrews 54 threaded intothe housing 42 and the ring 44 to engage the collar 34.

A lead wire 56 from the power supply 18 (and resistor 22) is received inan opening 60 in the housing 42 and is connected to the electrode 48 bya setscrew 62. The electrode shank 480 extends through the inner surfaceof the housing 42 and into the opening 60 for this purpose.

A counterbore 64 of a diameter substantially greater than that of thegun spray head 36 is provided in the housing 42 concentrically of thepaint stream axis. Immediately adjacent the point where it protrudesinto the counterbore 64, the electrode leg 48a, or shank, is bent at aright angle to extend in a direction downstream of and generallyparallel to the direction of particle flow. At a point downstream of thehorns 38, the electrode is reversely bent along a radius of curvature toform a leg 48b. The angle subtended by the legs 48a and 48b ispreferably within the range of from 30 to 60, so that the electrodeenters the spray pattern at about the same angle.

The radius of curvature of the bend 48:: is preferably within the rangeof from one-sixteenth inch to three-eighths inch to avoid theundesirable accumulation, or agglomeration, of large paint droplets onthe emitter electrode. This is an important factor in spray paintingsystems, inasmuch as the droplets of paint found to agglomerate on asharply bent electrode are, under certain circumstances, reentrained inthe paint stream and deposited on the workpiece, thereby resulting in apoor quality finish. Also, the diameter of the counterbore 64 should besufficiently large so that the curved portion 48c is well spaced fromthe paint stream axis. This not only reduces the turbulence introducedinto the paint particle stream by the emitter electrode, but alsofurther reduces the possibility of paint droplets accumulating on theelectrode.

The tip of the electrode 48 is preferably positioned on the axis of thepaint stream downstream from the spray nozzle (not shown) and the endsof the horns 38. But as noted above in connection with FIGS. 1A and 1Band as shown in FIG. 4, the electrode tip may actually be located evenlywith or upstream of the downstream ends of the horns. Although thespacing between the tip and the inner surfaces of the horns 38 will varyamong commercial guns, it is contemplated that the spacing ranges fromone-fourth inch to one-half inch to maintain a corona discharge bothduring and absent discharge of the atomized paint. Such spacing willproduce, for example, a particle charge mass ratio of 2-8 microcoulombsper gram at a typical voltage of 10 kilovolts, a ratio that has beenfound to be highly effective in coating workpieces with a high-qualityfinish.

The outwardly flared dielectric channel member 46 extends downstreamfrom the housing 42 concentrically of the atomized particle stream axis.In addition to guarding against sparking, it shields the gun spray head36, horns 38 and electrode 48 against the deposition of atomized paintparticles which have become dispersed and drift back towards the gun.

It also acts as a guide channel to shape the spray pattern and directthe stream of charged paint particles toward the workpiece. The guidingeffect of the channel member 46 results from the buildup of an electricpotential on the inner surface of the channel member caused by thedeposition of charged particles on the inner surface of the member. Thisbuildup creates an electric field which opposes the movement of otherlike-charged particles from the particle stream toward the channelmember so that, after a period of time, an equilibrium is establishedwhereby essentially all of the charged particles pass through thechannel member and are directed toward the workpiece. As a consequence,the charge carried by the particles is enhanced and a stronger spacecharge immediately downstream of the gun is produced than is attainedwithout a channel member. This further results in the conversion of morekinetic energy of each particle into potential energy in overcoming thespace charge field. Since the efficiency of an electrostatic coatingsystem increases with an increase in the charge carried by the coatingparticles, the efficiency of the system can be increased by use of thechannel member 46.

A second embodiment of the attachment unit, shown generally at 70, isshown in FIG. 3 in combination with an airless-type spray gun 72. Sincean airless-type spray gun has no air horns, the corona discharge isinitially established between the tip of the electrode 74 and the spraynozzle 76. Under certain operating conditions, the ion stream may alsoextend from the tip to the endmost surfaces of the spray head 7%. Thislatter path normally is followed only when paint particles are beingdischarged from the gun. The spacing of the electrode tip from thenozzle 76 is set at approximately one-fourth to one-half inch,preferably on or near the paint stream axis. The channel member 30functions in a manner similar to the channel member 46 and is likewisepreferably provided with an outwardly flared downstream end (not shown).

in this embodiment, the emitter electrode 74 is shown as a straightneedlelike member. Although the curved electrode and the straightelectrode are disclosed with an air-type gun and an airless-type gun,respectively, it is to be understood that the disclosure is made in thismanner purely for illustrative purposes and that the electrodes may beinterchanged as desired. Although the straight electrode interferes withthe atomized particle stream to a greater extent than does the curvedelectrode, a corona discharge is established directly to the gun end andhighly efficient coating of workpieces is obtained with a far simplerelectrode arrangement than is presently known in the prior art.Moreover, the straight electrode is particularly useful for applicationswhere the channel member cannot be used because of space or otherlimitations and there is danger of the curved electrode being broughtnear enough to the workpiece for sparking to occur.

in the schematic illustration of FIG. 4, a conventional,nonelect'rostatic gun is shown with an electrostatic converter unit 90mounted on it to provide an electrostatic painting system for paintingsolid workpieces positioned within a chamber 92. Upon supplying currentto the emitter electrode and discharging atomized paint particles fromthe spray nozzle, a stream of charged atomized paint particles movesthrough the channel member 94 and is exhausted into the dielectricchamber 92, creating a space charge field. For purposes of explanation,the chamber 92 is shown to contain a solid workpiece 96 of arbitraryshape suitably supported within the chamber.

As the charged paint particles are emitted from the gun into the chamber92, the workpiece 96 receives an even coating over its entire surface,since the space charge field in the chamber is effective to induce thecharged particles to seek a potential lower than the space chargepotential. As portions of the surface attain a higher charge, andthereby a greater potential, the attractive forces between other chargedparticles and the workpiece 96 will decrease and, assuming a dielectricworkpiece surface, the charged particles will tend to deposit on thosesurface portions at a lower potential. Of course, the gun may be usedwithout the chamber 92 to paint workpieces located in open areas,although the rapidity with which the workpieces will be coated will bereduced somewhat.

From the foregoing it is apparent that the invention provides animproved, far simpler electrostatic coating apparatus. Because of itslow operating potential capability, owing to short electrode spacing,the apparatus does not require the use of bulky insulation, heavy leadlines, large power packs, etc., and therefore is particularly adaptedfor converting conventional hand-held spray guns.

It will be understood by those skilled in the art that the embodimentsdescribed are intended to be merely exemplary, in that they aresusceptible of modification and variation without departing from thespirit and scope of the invention. For example, although the chargingapparatus has been disclosed in the form of a converter unit forconverting conventional nonelectrostatic spray guns to electrostaticpainting systems, it may also be incorporated into a spray-paintinginstallation as an integral part of the spray gun or its associatedequipment. All such modifications and variations, therefore, areintended to be included within the scope and spirit of the invention asdefined in the appended claims.

1. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, an emitter electrode spaced from the spray head,means for establishing a corona discharge in the space between theemitter electrode and the spray head and terminating at the spray head,and means for positioning the emitter electrode within the particlestream downstream of and in spaced relation to the spray head such thatthe principal component of the corona discharge extends in the upstreamdirection, whereby the particles pass through the corona discharge inopposed relation thereto.

2. Apparatus as defined in claim 1 wherein the spray head includes aspray nozzle for issuing the spray particles and wherein the coronadischarge terminates at least in part at the spray nozzle and at leastin part at the portion of the spray head immediately surrounding thespray nozzle.

3. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, the spray head including at least one air horn fordirecting a jet of air toward the particle stream, an emitter electrodedisposed downstream of and in spaced relation to the spray head, andmeans for establishing a corona discharge having its principal componentextending upstream in the space between the emitter electrode and thespray head so that the corona discharge terminates at least in part atthe air horn, whereby the particles pass through the corona discharge inopposed relation thereto.

4. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, an elongate needlelike emitter electrode positioneddownstream of and in spaced relation to the spray head, the emitterelectrode having a pointed tip disposed substantially on thelongitudinal axis of the particle stream, and means for establishing acorona discharge having its principal component extending upstream inthe space between the emitter electrode tip and the spray head, wherebythe particles pass through the corona discharge in opposed relationthereto.

5. Apparatus in accordance with claim 4 wherein the spray head includesat least one air horn for directing a jet of air toward the particlestream and wherein the corona discharge is established at least in partbetween the emitter electrode and air horn.

6. Apparatus as defined in claim 4 wherein the spray head includes aspray nozzle for issuing the spray particles and wherein the coronadischarge is established at least in part between the emitter electrodeand the spray nozzle.

7. Apparatus in accordance with claim 4 wherein the emitter electrode isso disposed as to extend into the particle stream in the upstreamdirection at an angle of from about 30 to 60 with respect to thelongitudinal axis of the particle stream so that the tip of the emitterelectrode is directed at least partly toward the spray head.

8. Apparatus in accordance with claim 7, wherein the shank of theemitter electrode is bent along a predetermined radius of curvature toextend back toward the spray head.

9. Apparatus as defined in claim 8 wherein said predetermined radius ofcurvature is within the range of from one-sixteenth inch tothree-eighths inch.

lit). Apparatus as defined in claim 8 wherein the tip of the emitterelectrode is spaced from the spray nozzle within the range of fromone-fourth inch to one-half inch.

111. Apparatus in accordance with claim 6 wherein the emitter electrodeis so disposed as to extend into the particle stream in the upstreamdirection at an acute angle with respect to the longitudinal axis of theparticle stream so that the tip of the emitter electrode is directed atleast partly toward the spray head.

12. Apparatus in accordance with claim 4 wherein the tip of the emitterelectrode is directed at least partly toward the spray head, whereby thestream of particles issuing from the spray head is directed inopposition to the direction of the emitter electrode tip.

13. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, an emitter electrode disposed downstream of and inspaced relation to the spray head, and means, including a high-potentialpower supply for maintaining the emitter electrode at a potential withinthe range of from 8 to 12 kilovolts and a conductor connecting the sprayhead to ground, for establishing a corona discharge having its principalcomponent extending upstream in the space between the emitter electrodeand the spray head, whereby the particles pass through the coronadischarge in opposed relation thereto.

14. An electrostatic spray charging apparatus for attachment to anonelectrostatic spray gun having an electrically conductive spray headfor discharging a stream of particles comprising: a housing membercomposed of dielectric material, an elongate emitter electrode having apointed tip carried by the housing member, means on the housing memberfor adjustably attaching the housing member to the nonelectrostatic gunto position the emitter electrode tip at a point spaced downstream fromthe spray head and substantially on the longitudinal axis of the spraystream, and means for maintaining a corona discharge between the tip ofthe emitter electrode and the spray head in a zone occupied by theparticle stream, the corona discharge having its principal componentextending in the upstream direction, whereby the spray particles passthrough the corona discharge in opposed relation thereto.

15. An electrostatic charging apparatus as defined in claim 14 whereinthe corona discharge terminates in part at the portion of the spray headimmediately surrounding the discharged stream of particles, and themeans for maintaining the corona discharge between the emitter electrodetip and the spray head includes a high-potential power supply and aconductor attachable to the spray head for connecting the spray head toa reference potential.

16. Apparatus as defined in claim 15 wherein the power supply provides apotential within the range of from 8 to 12 kilovolts.

17. Apparatus as defined in claim 14 wherein the emitter electrode isbent along a predetermined radius of curvature to form first and secondleg portions, the first leg portion being secured at its free end to thehousing and extending therefrom in the downstream direction; the secondleg portion extending in a direction having a substantial upstreamcomponent and having said pointed tip at its free end, whereby saidpointed tip is directed generally in opposition to the particle flowdirection when the housing is attached to the spray gun.

18. Apparatus as defined in claim 17 wherein the first leg portion ofthe emitter electrode is generally parallel with the longitudinal axisof the particle stream and wherein the angle subtended by the first andsecond leg portions is within the range of from 30 to 60.

19. Apparatus as defined in claim 18 wherein said predetermined radiusof curvature is within the range of from one-sixteenth inch tothree-eighths inch.

20. Apparatus according to claim 14 wherein the emitter electrodeincludes an elongated member forming an angle of between about 30 and 60with the longitudinal axis of the particle stream and having a tipportion located closest to the spray head.

21. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, an elongated needlelike emitter electrode having apointed tip, the tip being disposed substantially on the longitudinalaxis of the particle stream in downstream spaced relation to the sprayhead, the emitter electrode further being disposed so as to extend intothe particle stream in the upstream direction at an angle of from about30 to 60 with respect to the longitudinal axis of the particle stream sothat the tip of the emitter electrode is directed at least partly towardthe spray head, and means for establishing a corona discharge having itsprinci al component extending in the upstream direction in e spacebetween the emitter electrode and the spray head, whereby the particlespass through the corona discharge in opposed relation thereto.

22. In the method of electrostatically painting an object by means of acharged cloud of particles wherein paint is atomized upon beingdischarged from a spray head and formed into a particle spray stream inthe presence of a high concentration of atmospheric ions, theimprovement comprising the steps of:

positioning the tip of an emitter electrode at a point substantially onthe longitudinal axis of the particle spray stream and spaced downstreamfrom the spray head, and

establishing a potential difference between the emitter electrode tipand the spray head sufiicient to maintain a corona dischargetherebetween whose principal component is directed upstream through theportion of the particle spray stream between the electrode tip and thespray head, whereby the particles pass through the corona discharge inopposition to the principal component thereof.

2. Apparatus as defined in claim 1 wherein the spray head includes aspray nozzle for issuing the spray particles and wherein the coronadischarge terminates at least in part at the spray nozzle and at leastin part at the portion of the spray head immediately surrounding thespray nozzle.
 3. Apparatus for charging particles of coating materialcomprising: a spray gun having an electrically conductive spray head forissuing a stream of particles, the spray head including at least one airhorn for directing a jet of air toward the particle stream, an emitterelectrode disposed downstream of and in spaced relation to the sprayhead, and means for establishing a corona discharge having its principalcomponent extending upstream in the space between the emitter electrodeand the spray head so that the corona discharge terminates at least inpart at the air horn, whereby the particles pass through the coronadischarge in opposed relation thereto.
 4. Apparatus for chargingparticles of coating material comprising: a spray gun having anelectrically conductive spray head for issuing a stream of particles, anelongate needlelike emItter electrode positioned downstream of and inspaced relation to the spray head, the emitter electrode having apointed tip disposed substantially on the longitudinal axis of theparticle stream, and means for establishing a corona discharge havingits principal component extending upstream in the space between theemitter electrode tip and the spray head, whereby the particles passthrough the corona discharge in opposed relation thereto.
 5. Apparatusin accordance with claim 4 wherein the spray head includes at least oneair horn for directing a jet of air toward the particle stream andwherein the corona discharge is established at least in part between theemitter electrode and air horn.
 6. Apparatus as defined in claim 4wherein the spray head includes a spray nozzle for issuing the sprayparticles and wherein the corona discharge is established at least inpart between the emitter electrode and the spray nozzle.
 7. Apparatus inaccordance with claim 4 wherein the emitter electrode is so disposed asto extend into the particle stream in the upstream direction at an angleof from about 30* to 60* with respect to the longitudinal axis of theparticle stream so that the tip of the emitter electrode is directed atleast partly toward the spray head.
 8. Apparatus in accordance withclaim 7, wherein the shank of the emitter electrode is bent along apredetermined radius of curvature to extend back toward the spray head.9. Apparatus as defined in claim 8 wherein said predetermined radius ofcurvature is within the range of from one-sixteenth inch tothree-eighths inch.
 10. Apparatus as defined in claim 8 wherein the tipof the emitter electrode is spaced from the spray nozzle within therange of from one-fourth inch to one-half inch.
 11. Apparatus inaccordance with claim 4 wherein the emitter electrode is so disposed asto extend into the particle stream in the upstream direction at an acuteangle with respect to the longitudinal axis of the particle stream sothat the tip of the emitter electrode is directed at least partly towardthe spray head.
 12. Apparatus in accordance with claim 4 wherein the tipof the emitter electrode is directed at least partly toward the sprayhead, whereby the stream of particles issuing from the spray head isdirected in opposition to the direction of the emitter electrode tip.13. Apparatus for charging particles of coating material comprising: aspray gun having an electrically conductive spray head for issuing astream of particles, an emitter electrode disposed downstream of and inspaced relation to the spray head, and means, including a high-potentialpower supply for maintaining the emitter electrode at a potential withinthe range of from 8 to 12 kilovolts and a conductor connecting the sprayhead to ground, for establishing a corona discharge having its principalcomponent extending upstream in the space between the emitter electrodeand the spray head, whereby the particles pass through the coronadischarge in opposed relation thereto.
 14. An electrostatic spraycharging apparatus for attachment to a nonelectrostatic spray gun havingan electrically conductive spray head for discharging a stream ofparticles comprising: a housing member composed of dielectric material,an elongate emitter electrode having a pointed tip carried by thehousing member, means on the housing member for adjustably attaching thehousing member to the nonelectrostatic gun to position the emitterelectrode tip at a point spaced downstream from the spray head andsubstantially on the longitudinal axis of the spray stream, and meansfor maintaining a corona discharge between the tip of the emitterelectrode and the spray head in a zone occupied by the particle stream,the corona discharge having its principal component extending in theupstream direction, whereby the spray particles pass through the coronadischarge in opposed relation thereto.
 15. An electrostatic chargingapparaTus as defined in claim 14 wherein the corona discharge terminatesin part at the portion of the spray head immediately surrounding thedischarged stream of particles, and the means for maintaining the coronadischarge between the emitter electrode tip and the spray head includesa high-potential power supply and a conductor attachable to the sprayhead for connecting the spray head to a reference potential. 16.Apparatus as defined in claim 15 wherein the power supply provides apotential within the range of from 8 to 12 kilovolts.
 17. Apparatus asdefined in claim 14 wherein the emitter electrode is bent along apredetermined radius of curvature to form first and second leg portions,the first leg portion being secured at its free end to the housing andextending therefrom in the downstream direction; the second leg portionextending in a direction having a substantial upstream component andhaving said pointed tip at its free end, whereby said pointed tip isdirected generally in opposition to the particle flow direction when thehousing is attached to the spray gun.
 18. Apparatus as defined in claim17 wherein the first leg portion of the emitter electrode is generallyparallel with the longitudinal axis of the particle stream and whereinthe angle subtended by the first and second leg portions is within therange of from 30* to 60*.
 19. Apparatus as defined in claim 18 whereinsaid predetermined radius of curvature is within the range of fromone-sixteenth inch to three-eighths inch.
 20. Apparatus according toclaim 14 wherein the emitter electrode includes an elongated memberforming an angle of between about 30* and 60* with the longitudinal axisof the particle stream and having a tip portion located closest to thespray head.
 21. Apparatus for charging particles of coating materialcomprising: a spray gun having an electrically conductive spray head forissuing a stream of particles, an elongated needlelike emitter electrodehaving a pointed tip, the tip being disposed substantially on thelongitudinal axis of the particle stream in downstream spaced relationto the spray head, the emitter electrode further being disposed so as toextend into the particle stream in the upstream direction at an angle offrom about 30* to 60* with respect to the longitudinal axis of theparticle stream so that the tip of the emitter electrode is directed atleast partly toward the spray head, and means for establishing a coronadischarge having its principal component extending in the upstreamdirection in the space between the emitter electrode and the spray head,whereby the particles pass through the corona discharge in opposedrelation thereto.
 22. In the method of electrostatically painting anobject by means of a charged cloud of particles wherein paint isatomized upon being discharged from a spray head and formed into aparticle spray stream in the presence of a high concentration ofatmospheric ions, the improvement comprising the steps of: positioningthe tip of an emitter electrode at a point substantially on thelongitudinal axis of the particle spray stream and spaced downstreamfrom the spray head, and establishing a potential difference between theemitter electrode tip and the spray head sufficient to maintain a coronadischarge therebetween whose principal component is directed upstreamthrough the portion of the particle spray stream between the electrodetip and the spray head, whereby the particles pass through the coronadischarge in opposition to the principal component thereof.